1. Field of the Invention
The present invention relates to an energy transfer unit which is connected to a plurality of energy storage means to transfer the energy among the energy storage means mutually, and a charge unit and, more particularly, an energy transfer unit which is suitable for uniforming respective inter-terminal voltages of a plurality of energy storage means, and a charge unit and a power supply unit employing the energy transfer unit.
2. Description of the Related Art
As an energy transfer unit of this type, for example, the transfer unit 1061 disclosed in unexamined Japanese Patent Publication (KOKAI) Hei 7-322516 has been known in the conventional art. As shown in FIG. 30, a transfer unit 1061 is constructed to uniform storage energy of a plurality of capacitors C1 to C4 by transferring the storage energy in any one of the capacitors C1 to C4 to other capacitors. More particularly, the transfer unit 1061 comprises a series circuit of a choke coil L1 and a switch SW1 connected in parallel with a capacitor C1, a switch SW21 connected to a capacitor C2 via the choke coil L1, a series circuit of a choke coil L2 and a switch SW22 connected in parallel with the capacitor C2, a series circuit of a choke coil L3 and a switch SW31 connected in parallel with a capacitor C3, a switch SW32 connected to the capacitor C3 via the choke coil L2, and a switch SW4 connected to the capacitor C4 via the choke coil L3.
In the transfer unit 1061, for example, upon transferring the storage energy of the capacitors C4 to the capacitor C1, first the switch SW4 is controlled into the ON state. At this time, as shown in FIG. 30, the choke coil L3 is excited by a current I61. Then, a current I62 based on the exciting energy of the choke coil L3 flows by controlling simultaneously the switch SW4 and the switch SW31 into the OFF state and the ON state respectively to charge the capacitor C3. Then, the switch SW31 is controlled into the OFF state and then the switch SW32 is controlled into the ON state. Thus, a current I63 flows to excite the choke coil L2. In turn, a current I64 based on the exciting energy of the choke coil L2 flows by controlling simultaneously the switch SW32 and the switch SW22 into the OFF state and the ON state respectively to charge the capacitor C2. Then, the switch SW22 is controlled into the OFF state and then the switch SW21 is controlled into the ON state. Thus, a current I65 flows to excite the choke coil L1. Finally, a current I66 based on the exciting energy of the choke coil L1 flows by controlling simultaneously the switch SW21 and the switch SW1 into the OFF state and the ON state respectively to charge the capacitor C1. As a result, the storage energy of the capacitor C4 is transferred to the capacitor C1.
The transfer unit 1061 in the conventional art contains the problems described as follows.
In other words, in the transfer unit 1061, for example, upon transferring the energy from the capacitors C4 to the capacitors C3, the switch SW4 and the switch SW31 must be controlled simultaneously into the OFF state and the ON state. In this case, if the switch SW31 is controlled into the ON state prior to the OFF state of the switch SW4, the capacitors C3 and C4 are short-circuited via the switches SW4 and SW31, resulting in loss of the storage energy of both capacitors C3 and C4. On the contrary, if the switch SW4 is controlled into the OFF state prior to the ON state of the switch SW31, a very high voltage is generated across the switch SW4, resulting in failure of the switch SW4. In this manner, in the transfer unit 1061 in the conventional art, there is such a problem that, even if ON/OFF control timings of the switches S1 to SW4 are deviated slightly, short-circuit of the circuit and the failure of the circuit parts are brought about and also the energy cannot be transferred.
Also, in transferring the energy from the capacitor C4 to the capacitor C1, ON/OFF of the switches SW4 to SW1 must be controlled many times at a precise timing. Hence, in the transfer unit 1061, there is also such a problem that the control of the switches becomes complicated.
Further, in order to transfer the energy among four capacitors C1 to C4, six switches SW1 to SW4 must be used in the transfer unit 1061. In this case, if the situation that the energy is transferred among a number of capacitors is considered, the number of switches becomes about twice the number of capacitors. For this reason, in the transfer unit 1061 in the conventional art, there is also such a problem that, since the number of switches is increased, the transfer unit becomes expensive and the size of the unit is also increased.
A battery for vehicle and a charge unit for charging the battery have been developed swiftly in recent years. As one of the charge units of this type, a charge unit 1041 shown in FIG. 31 has already been developed. The charge unit 1041 is constructed to charge a battery BAT, which is constructed by series-connecting a plurality of electric double layer capacitors C1 to C4, for example, with high efficiency in compliance with the simply-constructed chopper system. In this charge unit 1041, when the control circuit (not shown) controls the switching element 1042 into the ON state, the charge current I11, which is generated based on the input voltage VIN such as the pulsating current obtained by rectifying the AC, the stabilized DC voltage, etc., flows through the current path comprising the switch element 1042, the choke coil 1043, and the battery BAT to thus charge the battery BAT. In contrast, when the switching element 1042 is controlled into the OFF state, the fly-wheel current IF flows through the current path comprising the choke coil 1043, the battery BAT, and the commutating diode 44, based on the energy stored in the choke coil 1043 during the flow of the charge current I11, whereby the battery BAT is also charged in this period. In this manner, in the charge unit 1041, the battery BAT can be continuously charged by the charge current I11 and the fly-wheel current IF during both periods of the ON period and the OFF period by ON/OFF-controlling the switch element 1042.
However, the charge unit 1041 in the conventional art contains the problems described as follows. In other words, in the charge unit 1041, the battery BAT is charged by passing the charge current I11 and the fly-wheel current IF through the capacitors C1 to C4 in series during both periods of the ON period and the OFF period of the switching element 1042. Therefore, the inter-terminal voltages of the capacitors C1 to C4 are not always maintained at the uniform voltage, rather they have a tendency to vary from each other. Meanwhile, the storage energy of the capacitor is in proportion to square of the inter-terminal voltage of the capacitor. Accordingly, even if the inter-terminal voltages are changed slightly, the storage energy of a plurality of capacitors C1 to C4 are changed considerably. Thus, such a situation occurs that, although the battery BAT has been charged sufficiently in due course, actually such battery BAT cannot discharge the enough electrical energy. For this reason, if the battery BAT for the electric vehicle is charged by the charge unit 1041, there is a possibility that the run-down of the battery is brought about after the vehicle travels merely within a short distance. In this manner, in the charge unit 1041 in the conventional art, there is such a problem that it is difficult to charge effectively sufficiently the storage means that is constructed by series-connecting a plurality storage elements. Further, there is another problem that, if the storage means is charged under the condition that the inter-terminal voltages of a plurality of storage elements are fluctuated, the lifetime of the storage elements is shortened.